A spectral sensitization process in silver halide photography is known as a means for extending light-sensitive wavelengths of a silver halide photographic emulsion from the intrinsic sensitivity region of silver halides toward longer wavelengths, and has served as an important technique. The sensitizable region has come to be extended to encompass infrared wavelengths, and is capable of being controlled by the skilled artisan by properly choosing sensitizing dyes having particular chemical structures so as to suit a particular need.
However, the popular demand for silver halide photographic emulsions having higher sensitivity continues to grow, and great efforts are being made by those engaged in research and development to enhance the sensitivity of emulsions. Under these circumstances, it is a natural consequence to strive for the development of novel sensitizing dyes capable of imparting higher spectral sensitivity to silver halide photographic emulsions.
One method employing sensitizing dyes to give higher sensitivity to emulsions consists of increasing light absorptivity of the emulsions, and sensitizing dyes developed in accordance with this idea are described in U.S. Pat. Nos. 3,622,317, 3,976,493 and 3,976,640. Another method consists of the exclusion of desensitization factors resulting from sensitizing dyes. As a means of carrying out this idea, supersensitization or the like is commonly performed, and many supersensitizing combinations of spectral sensitizing dyes are known. The desensitization factors mentioned above include development inhibition caused by sensitizing dyes. To counter this effect, a development accelerator often is used in conjunction with such sensitizing dyes.
In many cases, however, common spectral sensitizing dyes cannot coexist with other ingredients contained in a silver halide emulsion. Accordingly, a simple simultaneous use of a development accelerator and sensitizing dyes cannot always achieve the desired result. This inability to coexist is theorized to result from competition between sensitizing dyes and other chemical ingredients for occupying positions on the individual surfaces of silver halide grains. A general idea and compound examples proposed with the intention of eliminating this competitive phenomenon between a nucleating agent and sensitizing dyes are described in Japanese Patent Application (OPI) Nos. 9433/72 and 9678/72 (the term "OPI" as used herein means an "unexamined published application"), U.S. Pat. No. 3,718,470, and Research Disclosure No. 15162 (November 1976). However, examples which illustrate success in eliminating this competitive phenomenon are absent in the above cited literature, and no description of the attainment of spectral sensitivity higher than that obtained when using conventional sensitizing dyes is described therein.